CHICAGO (AP) - A study of heterosexual couples in Africaconcludes that the chance of catching the AIDS (news - web sites) virus from a singlesexual encounter with an infected person is one in 588.

This risk is calculated for people who do not use condoms andwho have sex regularly with one infected partner.

Earlier estimates from North America and Europe vary but havegenerally placed the risk at about one in 1,000 for heterosexuals.

In this study, researchers followed 174 sexually monogamouscouples in Rakai, Uganda, in which one partner had HIV (news - web sites) and theother did not. They were given condoms but usually did not usethem. Typically the couples had sex nine or 10 times a month, andover time, 38 people became infected.

Earlier data from the same research team showed that the risk ofpeople transmitting HIV is slight if the amount of virus in theirbloodstream is low. Those findings have encouraged the belief thatthe wide use of AIDS drug combinations, which make virus levelsfall dramatically, will slow the spread of the disease.

The latest figures were presented by Dr. Ronald H. Gray of JohnsHopkins University at the Eighth Annual Retrovirus Conference inChicago, which concluded Thursday.

Among the findings:

-Infected teen-agers are three times more likely than peopleover 40 to spread HIV to others during each sexual encounter. Thisdifference cannot be explained by the fact that young people aremore sexually active.

-The risk that an HIV-infected woman will transmit the virus toan uninfected man is one in 454. For an infected man to anuninfected woman, it is one in 769. This difference is not largeenough to be statistically meaningful, and many have assumed thatHIV spreads more readily from men to women than vice versa.

-The risk of spread depends greatly on how much virus peoplecarry. In those whose level of virus is less than 1,700 copies permilliliter of blood, the risk is one in 10,000. When levels areover 38,500, risk is one in 294.

-The risk of transmission appears to be the same for differentsubtypes of virus. Some have speculated that AIDS is much moreprevalent in Africa because a different variety of the virusdominates there.

-None of the circumcised men in the study caught HIV. Someexperts have raised the possibility of promoting circumcision as away to control the epidemic.

Whether the transmission risk is the same among couples outsideAfrica is unclear, especially since virus levels may be higher inAfrica, where so few infected people get treated. However, Dr.Helene Gayle, AIDS chief at the U.S. Centers for Disease Controland Prevention (news - web sites), said the latest data at least offer a generalestimate of this risk.

I suppose the more studies take place, the more accurate the single sexual encounter risk will be. One in 588 now and interesting to see that the female-to-male transmission rate is higher than male-to-female (albeit insignificantly).Admittedly, this is just one study but it goes against previous assumptions.

Also going against previous assumptions is the article I've posted below, which appeared on the British HIV Association last October, and which cites "single contact studies in commercial sex workers which have shown transmission rates of 3.1% - 8.2% from women to men, and rates in excess of 50% per episode in men to women".

This percentage game is wild! I sat down and worked out my risk (one-off encounter, condom break, partner of unknown HIV status, post-six-month seroconversion) as being anywhere between 0.0000001 and 0.000000000001. Which was enough to make me start looking for explanations other than HIV infection.

Speaking of which, I was also very interested to read that "several studies in commercial sex workers and other individuals repeatedly exposed to HIV have shown that HIV-specific cytotoxic T-lymphocyte responses to HIV are detectable in these individuals, despite no evidence of HIV infection". Does anyone have any info on this? I've had a bit of a search but come up with nothing. I'm just wondering whether there is any symptomatological association with this response.

Full article for anyone who's interested.

aidsmap news archive: HIV risk from single contact may be higher than thought

The risk of HIV transmission from activities like unprotected anal intercourse with new partners may be dangerously underestimated by long-term studies of serodiscordant partners, a leading researcher into the sexual transmission of HIV said today.

Speaking at the Fifth International Congress on Drug Therapy in HIV Infection, Dr Pietro Vernazza of the Cantonal Hospital, St Gallen, Switzerland, said:

“We’re citing these studies all the time, but I think it’s time we re-considered other data”.

Although long-term partner studies have shown that the risk of male to female or female to male transmission from a single episode of sexual intercourse may be as little as 0.3%, Vernazza points to single contact studies in commercial sex workers which have shown transmission rates of 3.1% - 8.2% from women to men, and rates in excess of 50% per episode in men to women.

He argues that long-term partner studies do not factor in the protective effect of any HIV-specific immunity that develops in partners who are repeatedly exposed to HIV but remain uninfected. Several studies in commercial sex workers and other individuals repeatedly exposed to HIV have shown that HIV-specific cytotoxic T-lymphocyte responses to HIV are detectable in these individuals, despite no evidence of HIV infection.

“The highest risk of transmission probably lies in the first few contacts” said Pietro Vernazza.

Risk of transmission during structured treatment interruptions

Vernazza also presented a case history which is likely to have a strong influence on advice about structured treatment interruptions – a case of sexual transmission during an STI by a man with previously undetectable viral load.

The man, who had had undetectable viral load prior to the treatment interruption, had been having unprotected sexual intercourse with his female partner prior to the STI. Two weeks after the STI, his female partner reported an acute viral illness, and tests revealed that she had become infected with HIV.

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